In refrigeration systems which are used in the food industry to refrigerate fresh food or frozen foods, it is essential from time to time during the period of a day to defrost the refrigeration coils of the evaporators which become clogged up by the build-up of ice thereon during the freezing cycle and which obstruct the passage of air whereby to supply the display cases or refrigerated enclosures to maintain foodstuff refrigerated. For example, in refrigerated display cases, operating at medium temperature range, for meat, dairy, fruits, etc., the refrigeration coils of the evaporators may undergo three defrost cycles of 12 minutes during a 24 hour period. On the other hand, in refrigerated enclosures which are provided with doors to store frozen foods, the defrost cycle may be longer and usually will last for about 20 minutes.
There are essentially three ways to defrost refrigeration coils. One utilizes an air defrost method wherein fans are utilized to direct a warm air stream against the refrigeration coils while the refrigerant is cut out from circulating through the coils. This results in fairly lengthy defrost cycles and which can last up to about 40 minutes. Another method is to pass cooled gas through the refrigeration coils with the gas being taken from the top of the refrigerant reservoir at a temperature of from about 80.degree. F. to 90.degree. F. Because the gas is circulated slowly within the refrigerant coils due to the pressure in the system, the defrost cycle is fairly lengthy. Another system utilizes hot gas defrost as briefly described in my U.S. Pat. No. 5,673,567 entitled "Refrigeration System with Heat Reclaim and Method of Operation". In that system hot gas from the compressor discharge line is fed to the refrigerant coil via a valving circuit and back into the liquid manifold to mix with the refrigerant liquid. This method of defrost usually takes about 12 minutes for defrosting evaporators associated with meat display cases and about 22 minutes for defrosting frozen food enclosures.
Because of the lengthy defrost cycles of existing defrost systems, the refrigeration system as well as the foodstuff is subject to adverse effects. For example, during the defrost cycle, compressor head pressure is increased and the energy cost increases. Also, the compressors are subjected to overheating after the defrost cycle when the hot gas comes back through the suction header and the life thereof is therefore reduced. Known defrost systems also operate at high pressure, such as the system described in my above-mentioned U.S. Patent, and this is due to the fact that the refrigerant liquid in the liquid line is at substantially the same pressure as the gas in the hot gas manifold of the compressors. The compressors therefore need to work harder to pump the hot gas through the evaporators at higher pressure to have a pressure differential of about 30 p.s.i. across the evaporator.
The foodstuff which is to be refrigerated and which is placed in display cases, such as fresh packaged meat placed in trays and wrapped with plastic film, fresh vegetables and seafoods, milk, drinks, cold-cuts, or like prepared meats, etc., it is desirable to maintain these at a medium refrigerated temperature. However, some of these products have been found to deteriorate during the defrost cycle. As an example only, when poultry is displayed in such cases and packaged in a plastic wrap, it has been found that the flesh of the poultry may change color when subjected to an important temperature variation. With fish, the freshness of the fish deteriorates, although this is not visually apparent. Cheeses can also deteriorate more rapidly during the defrost cycle and milk will not retain its freshness as long. In the trade, sometimes the butchers will rewrap the meat product which discolor and this is known not to be sanitary. Furthermore, after one and a half days of exposure in display counters, meat in such refrigerators has to be reprocessed into ground meat or discarded. Accordingly, it can be appreciated that expensive meat such as tenderloins, etc. when reprocessed into ground meat will demand a much lower price. Because of Governmental health regulations and laws, it is required that many of these food products be destroyed after having been placed in a refrigerator display case for a certain period of time.
With frozen foodstuff adverse effects are also produced. Because the defrost cycles are fairly long, usually 20 minutes, the frozen food packages develop humidity. As an example only with frozen ice-cream, often ice crystals will form on the container as well as inside the container. The effect of having ice crystal build-up on the outside of the container obstructs the label and further makes that container unattractive when left in the refrigerator for long periods of time. Because ice crystals have also built-up on the inside of the containers, the ice-cream will be subject to faster deterioration. In frozen food cabinets or enclosures, the temperature is expected to be maintained at -10.degree. F. but during the defrost cycle, and particularly when doors to the enclosures are open, heat will rise into the enclosure as the defrost coils are being defrosted and defrost air is pushed into the cabinet.